Heretofore, there have been used oxygen sensors for detecting oxygen concentration in exhaust gases in order to purify exhaust gases by controlling the air-to-fuel ratio of an internal combustion engine, various combustion machines and the like.
The oxygen sensor produces an electromotive force in accordance with the difference between the oxygen concentration in a detection gas, such as an exhaust gas and the like, and the oxygen concentration in a reference gas. Since the electromotive force abruptly changes at the point of the theoretical air-to-fuel ratio, the air-to-fuel ratio of the detection gas can be obtained by measuring the electromotive force.
As the oxygen sensor element used for the above-described oxygen sensor, there is a sensor element in which electrodes are provided at both surfaces of an oxygen-ion-conductive solid electrolyte consisting of, for example, zirconia. One of the electrodes is exposed to the detection-gas side, and another electrodes is exposed to the reference-gas side.
There have been developed techniques for protecting the electrode at the detection-gas side of the above-described oxygen sensor element, and promoting the oxidation reaction of gas components, such as, CO, HC, H.sub.2 and the like, performed at the electrode to detect the air-to-fuel ratio with an excellent accuracy. There has been proposed, for example, an oxygen sensor in which a porous second coated layer is formed on the surface of the detection-gas-side electrode using Al.sub.2 O.sub.3, and a first coated layer is formed on the surface of the second coated layer using Al.sub.2 O.sub.3 which carries a Pt catalyst for promoting oxidation reaction (refer to JP Patent Kokoku Publication No. 57-34900 (1982)).